1. Technology Field
The present invention relates to an encryption technology, and more particularly to the technology for giving the compressed encryption functionality to integer-based homomorphic encryption schemes.
2. Description of the Related Art
Homomorphic encryption scheme is a form of encryption which allows operations to be carried out on encrypted data.
Recent researches on designs of the homomorphic encryption have been focused on realization of very large size of a public key which is used in conventional schemes. A research which should be accompanied along with those researches is to reduce cost for storing the encrypted data. Since an application environment which is expected to use homomorphic encryptions requires a serve to store high-volume data such as cloud services or big-data services, it is very important to develop not only techniques for reducing the length of a public key using fixed special resources but also techniques for reducing the size of encrypted data which is expected to be very variable and large.
Most of the techniques proposed to date are configured to encrypt only 1 bit information in a ciphertext. Thus, n(n is predetermined natural number) numbers of ciphertexts are generated for n bit encryption which results in increases in cost for the same level to store those data. Recently, the scheme to encrypt multiple bit information in a single ciphertext has been developed. An existing compressed encryption scheme allows the compressed encryption for l bit plaintext information in a single ciphertext when a compression ratio l is determined and then parameters which are corresponding thereto are determined in the design of the homomorphic encryption scheme. About n/l numbers of ciphertexts are generated for the encryption of n bit information so that the storage space efficiency is improved by about l times compared to those techniques which perform the encryption in units of bit. However, this existing compressed encryption scheme cannot use the parameters which are used for other integer-based schemes but should use parameters satisfying certain conditions for the compressed encryption.